Electro-optical measurement of hysteresis in interferometric modulators

ABSTRACT

Disclosed herein are methods and apparatus for testing interferometric modulators. The interferometric modulators may be tested by applying a time-varying voltage stimulus and measuring the resulting reflectivity from the modulators.

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 11/073,295entitled “ELECTRO-OPTICAL MEASUREMENT OF HYSTERESIS IN INTERFEROMETRICMODULATORS”, filed Mar. 4, 2005 which claims priority to U.S.Provisional Application No. 60/613,537, filed on Sep. 27, 2004, all ofwhich are hereby incorporated by reference in their entirety.

BACKGROUND

1. Field of the Invention

The field of the invention relates to microelectromechanical systems(MEMS).

2. Description of the Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. An interferometricmodulator may comprise a pair of conductive plates, one or both of whichmay be transparent and/or reflective in whole or part and capable ofrelative motion upon application of an appropriate electrical signal.One plate may comprise a stationary layer deposited on a substrate, theother plate may comprise a metallic membrane separated from thestationary layer by an air gap. Such devices have a wide range ofapplications, and it would be beneficial to utilize and/or modify thecharacteristics of these types of devices so that their features can beexploited in improving existing products and creating new products thathave not yet been developed. In order to ensure high quality, accurateand convenient methods for testing the operation of such MEMS devicesmay be employed in the manufacturing process. Further development ofsuch methods is needed.

SUMMARY OF CERTAIN EMBODIMENTS

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Certain Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

One embodiment comprises a method of testing a plurality ofinterferometric modulators, comprising applying a triangular voltagewaveform to the interferometric modulators and detecting reflectivity oflight from the interferometric modulators.

Another embodiment comprises a method of determining electricalparameters for driving a plurality of inteferometric modulators,comprising applying a time-varying voltage stimulus to theinterferometric modulators, detecting reflectivity of light from theinterferometric modulators, and determining one or more electricalparameters from the reflectivity of light in response to thetime-varying voltage stimulus, the electrical parameters indicative ofelectrical parameters sufficient to cause a change in state of theinterferometric modulators.

Another embodiment comprises a method of testing a plurality ofinterferometric modulator structures, comprising applying a time-varyingvoltage waveform to the interferometric modulators, detectingreflectivity of light from the interferometric modulators, determiningreflectivity of light as a function of voltage, and identifying theplurality of interferometric modulators as of sufficient quality for usein a display based on the determining.

Another embodiment comprises a system for testing a plurality ofinterferometric modulators, comprising an illumination source adapted toprovide incident light to a plurality of interferometric modulators, avoltage source adapted to apply enough voltage to the interferometricmodulators so as to change their state, and an optical detector adaptedto detect light reflected from the plurality of interferometricmodulators.

Another embodiment comprises a method of testing a plurality ofinteferometric modulators, comprising applying a time-varying voltagewaveform to the interferometric modulators, detecting reflectivity oflight from the interferometric modulators, repeating the applying anddetecting steps one or more times, averaging at least a portion of thedetected reflectivity, and determining one or more electrical parametersfrom the averaged reflectivity.

Another embodiment comprises a method of testing a color interferometricmodulator display, the display comprising a plurality of sub-pixeltypes, each sub-pixel type corresponding to a different color, themethod comprising applying a time-varying voltage waveform to onesub-pixel type, detecting reflectivity of light from the sub-pixel type,determining one or more electrical parameters from the detecting, andrepeating the applying, detecting, and determining steps for anothersub-pixel type.

Another embodiment comprises a method of testing a plurality ofinterferometric modulators, comprising a means for applying atime-varying voltage waveform to the interferometric modulators, a meansfor detecting reflectivity of light from the interferometric modulators,and a means for determining one or more parameters based on the detectedreflectivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a released position and amovable reflective layer of a second interferometric modulator is in anactuated position.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIG. 6A is a cross section of the device of FIG. 1.

FIG. 6B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 6C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7 is a plot of applied voltage and reflectivity response as afunction of time when a triangular voltage waveform is applied to aninterferometric modulator.

FIG. 8 is a plot of reflectivity from an interferometric modulator as afunction of applied voltage.

FIG. 9 is a flowchart depicting methods for testing interferometricmodulators by applying a time-varying voltage stimulus and measuringreflectivity response.

FIG. 10 is a flowchart depicting methods for determining actuation andrelease voltages in interferometric modulators by normalizingreflectance.

FIG. 11 is an illustration of an apparatus for testing aninterferometric modulator by applying a voltage stimulus and measuringreflectivity response.

FIG. 12 is an illustration of an apparatus for testing aninterferometric modulator by applying a voltage stimulus and measuringreflectivity response using in-line lighting and detection.

FIG. 13 is an illustration of an interferometric modulator array withtest areas identified.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As described in more detail below, interferometricmodulators exhibit a hysteresis characteristic that affects the way thatthe interferometric modulators can be driven. Thus, one test of whethermanufactured interferometric modulators are appropriate for use in adisplay is to determine whether a desired hysteresis characteristic isobserved. Furthermore, measurement of the hystersis characteristics ofinterferometric modulators can provide the appropriate electricalparameters for use in driving the interferometric modulators.Accordingly, described below are methods and devices for testing thehysteresis characteristics of interferometric modulators by applying atime-varying voltage stimulus to the modulators and detecting theresponse of reflectance from the modulators.

As will be apparent from the following description, embodiments may beprovided in any device that is configured to display an image, whetherin motion (e.g., video) or stationary (e.g., still image), and whethertextual or pictorial. More particularly, it is contemplated that theembodiments may be implemented in or associated with a variety ofelectronic devices such as, but not limited to, mobile telephones,wireless devices, personal data assistants (PDAs), hand-held or portablecomputers, GPS receivers/navigators, cameras, MP3 players, camcorders,game consoles, wrist watches, clocks, calculators, television monitors,flat panel displays, computer monitors, auto displays (e.g., odometerdisplay, etc.), cockpit controls and/or displays, display of cameraviews (e.g., display of a rear view camera in a vehicle), electronicphotographs, electronic billboards or signs, projectors, architecturalstructures, packaging, and aesthetic structures (e.g., display of imageson a piece of jewelry). MEMS devices of similar structure to thosedescribed herein can also be used in non-display applications such as inelectronic switching devices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as thereleased state, the movable layer is positioned at a relatively largedistance from a fixed partially reflective layer. In the secondposition, the movable layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable and highly reflective layer 14 ais illustrated in a released position at a predetermined distance from afixed partially reflective layer 16 a. In the interferometric modulator12 b on the right, the movable highly reflective layer 14 b isillustrated in an actuated position adjacent to the fixed partiallyreflective layer 16 b.

The fixed layers 16 a, 16 b are electrically conductive, partiallytransparent and partially reflective, and may be fabricated, forexample, by depositing one or more layers each of chromium andindium-tin-oxide onto a transparent substrate 20. The layers arepatterned into parallel strips, and may form row electrodes in a displaydevice as described further below. The movable layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes 16 a, 16 b) deposited on top ofposts 18 and an intervening sacrificial material deposited between theposts 18. When the sacrificial material is etched away, the deformablemetal layers are separated from the fixed metal layers by a defined airgap 19. A highly conductive and reflective material such as aluminum maybe used for the deformable layers, and these strips may form columnelectrodes in a display device.

With no applied voltage, the cavity 19 remains between the layers 14 a,16 a and the deformable layer is in a mechanically relaxed state asillustrated by the pixel 12 a in FIG. 1. However, when a potentialdifference is applied to a selected row and column, the capacitor formedat the intersection of the row and column electrodes at thecorresponding pixel becomes charged, and electrostatic forces pull theelectrodes together. If the voltage is high enough, the movable layer isdeformed and is forced against the fixed layer (a dielectric materialwhich is not illustrated in this Figure may be deposited on the fixedlayer to prevent shorting and control the separation distance) asillustrated by the pixel 12 b on the right in FIG. 1. The behavior isthe same regardless of the polarity of the applied potential difference.In this way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application. FIG. 2is a system block diagram illustrating one embodiment of an electronicdevice that may incorporate aspects of a display application. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array controller 22. In one embodiment, the array controller 22includes a row driver circuit 24 and a column driver circuit 26 thatprovide signals to a pixel array 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the released state to the actuated state.The potential difference required to cause actuation of aninterferometric modulator may be referred to as the “actuationpotential.” When the voltage is reduced from that value, the movablelayer maintains its state as the voltage drops back below 10 volts. Inthe exemplary embodiment of FIG. 3, the movable layer does not releasecompletely until the voltage drops below 2 volts. The potentialdifference at which and actuated interferometric modulator releases maybe known as the “release potential.” There is thus a range of voltage,about 3 to 7 V in the example illustrated in FIG. 3, where there existsa window of applied voltage within which the device is stable in eitherthe released or actuated state. This is referred to herein as the“hysteresis window” or “stability window.” For a display array havingthe hysteresis characteristics of FIG. 3, the row/column actuationprotocol can be designed such that during row strobing, pixels in thestrobed row that are to be actuated are exposed to a voltage differenceof about 10 volts, and pixels that are to be released are exposed to avoltage difference of close to zero volts. After the strobe, the pixelsare exposed to a steady state voltage difference of about 5 volts suchthat they remain in whatever state the row strobe put them in. Afterbeing written, each pixel sees a potential difference within the“stability window” of 3-7 volts in this example. This feature makes thepixel design illustrated in FIG. 1 stable under the same applied voltageconditions in either an actuated or released pre-existing state. Sinceeach pixel of the interferometric modulator, whether in the actuated orreleased state, is essentially a capacitor formed by the fixed andmoving reflective layers, this stable state can be held at a voltagewithin the hysteresis window with almost no power dissipation.Essentially no current flows into the pixel if the applied potential isfixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with interferometric modulator displays.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Releasing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias.)

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or released states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and releases the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and release pixels (2,1) and (2,3). Again,no otherpixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used withinterferometric modulator displays.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 6A-6C illustrate three different embodiments of themoving mirror structure. FIG. 6A is a cross section of the embodiment ofFIG. 1, where a strip of metal material 14 is deposited on orthogonallyextending supports 18. In FIG. 6B, the moveable reflective material 14is attached to supports at the corners only, on tethers 32. In FIG. 6C,the moveable reflective material 14 is suspended from a deformable layer34. This embodiment has benefits because the structural design andmaterials used for the reflective material 14 can be optimized withrespect to the optical properties, and the structural design andmaterials used for the deformable layer 34 can be optimized with respectto desired mechanical properties. The production of various types ofinterferometric devices is described in a variety of publisheddocuments, including, for example, U.S. Published Application2004/0051929. A wide variety of well known techniques may be used toproduce the above described structures involving a series of materialdeposition, patterning, and etching steps.

As described above, the hysteresis characteristics of interferometricmodulators affect the way that the interferometric modulators can bedriven and provide a stability or “memory” voltage window in whichinterferometric modulators can maintain their state with little powerconsumption. Thus, one test of whether manufactured interferometricmodulators are appropriate for use in a display is to determine whethera desired hysteresis characteristic is observed. Furthermore, measuringof the hystersis characteristics of interferometric modulators canprovide the appropriate electrical parameters for use in driving theinterferometric modulators. For example, the appropriate bias potential,V_(bias) (also referred to herein as “V_(b)”), for use in drivinginterferometric modulators as described above can be determined bymeasuring hysteresis characteristics. In addition, once the hysteresischaracteristics of an interferometric modulator array is known, suitabledriving schemes other than that described above can be formulated.Accordingly, described below are methods and devices for testing thehysteresis characteristics of interferometric modulators.

In one embodiment, the hysteresis characteristics may be determined bymeasuring the reflectance from one or more interferometric modulators inresponse to a time-varying voltage stimulus applied to theinterferometric modulators. The voltage stimulus may be such that theinterferometric modulators are varied between an actuated and anon-actuated state. The detection of reflectance from theinterferometric modulators will thus provide an indication of what statethe interferometric modulators are in because the reflectance frominterferometric modulators are different between actuated andnon-actuated states. In one embodiment, in an actuated state, thereflectance will be low and in a non-actuated state, the reflectancewill be high.

In one embodiment, the time-varying voltage stimulus is a triangularvoltage waveform. The amplitude of the triangular voltage waveform mayadvantageously be chosen so that it exceeds the voltage necessary toinduce actuation of an interferometric modulator. In one embodiment, theamplitude of the waveform is between about 1.0 to about 2.0 times thevoltage necessary to induce interferometric modulator actuation. In oneembodiment, the amplitude of the waveform is about 1.25 times thevoltage necessary to induce interferometric modulator actuation. In oneembodiment, the frequency of the voltage waveform is slow enough so thatthe interferometric modulator response approximates a DC voltageresponse. In one embodiment, the frequency is less than about 20 Hz. Inone embodiment, the frequency is between about 1 Hz and about 15 Hz. Inone embodiment, the frequency is about 2.5 Hz. Those of skill in the artwill appreciate that time-varying voltage waveforms other thantriangular may be applied to determine the hysteresis characteristics ofinterferometric modulators.

One example of an applied triangular voltage waveform and reflectanceresponse is depicted in FIG. 7. FIG. 7 shows a plot 100 of a triangularvoltage waveform applied to an interferometric modulator and a plot 102of the resulting reflectance from the interferometric modulator as afunction of time. Prior to application of the triangular voltagewaveform (e.g., from 0 to 150 ms in FIG. 7), the voltage isapproximately 0 and the reflectance from the interferometric modulatoris high. The high reflectance is due to the interferometric modulatorbeing in an unactuated state. Upon commencement of applying thetriangular voltage waveform, the voltage is ramped in a positivedirection (e.g., starting at 150 ms in FIG. 7). Initially, thereflectance remains high and relatively constant as the voltage isramped. Upon reaching a high enough voltage (e.g., approximately 7.3 Vin FIG. 7), the reflectance level suddenly drops due to theinterferometric modulator switching to an actuated state. The voltage atwhich this transition occurs may be referred to as the positiveactuation potential (V_(pa)) 104. As the voltage is increased further,no change in reflectance is observed.

Next, the voltage is ramped in a negative direction (e.g., starting at350 ms in FIG. 7). The reflectance remains low and relatively constantuntil the voltage is low enough, when the reflectance will suddenly jumpback to a high level (e.g., at approximately 4.3 V in FIG. 7) due to theinterferometric modulator switching back to a non-actuated state. Thevoltage at which this transition occurs may be referred to as thepositive release potential (V_(pr)) 106. The voltage may be decreasedfurther into negative values with little change in reflectance. When thevoltage reaches a negative enough value, another sudden decrease inreflectance is observed (e.g., at approximately −7.8 V in FIG. 7) due tothe interferometric modulator re-actuating. The voltage at which thistransition occurs may be referred to as the negative actuation potential(V_(na)) 108. As the voltage is decreased further, no change inreflectance is observed.

Finally, the voltage is again ramped in a positive direction (e.g.,starting at 750 ms in FIG. 7). No significant change in reflectance isobserved until the voltage increases to a sufficient level (e.g., atapproximately −5 V in FIG. 7) at which point the reflectance suddenlyincreases due to transition of the interferometric modulator to anon-actuated state. The voltage at which this transition occurs may bereferred to as the negative release potential (V_(nr)) 110. FIG. 7demonstrates that two memory windows 112 and 114 exist. Voltages appliedwithin the windows 112 and 114 will not cause a change ininterferometric modulator state.

The four parameters that characterize an interferometric modulator'shysteresis characteristic, V_(pa), V_(pr), V_(na), and V_(nr), may beused to determine the electrical parameters necessary to drive theinterferometric modulator. For example, it is desirable that pulses toactuate the interferometric modulator be such that a voltage in excessof V_(pa) or below V_(na) is applied. Similarly, to release aninterferometric modulator, it is desirable that pulses with voltagesbetween V_(pr) and V_(nr) be applied. At all other times, it isdesirable that the applied drive voltage be within the two memorywindows so that the interferometric modulator does not change state.

As an alternative to the four parameters discussed above, aninterferometric modulator's hystersis characteristic may be defined by abias potential (V_(b)), an offset potential (V_(off)), and a singlememory window (ΔV_(mem)). While the hysteresis characteristic of aninterferometric modulator is substantially symmetric between positiveand negative potentials, the symmetry is usually not perfect. Forexample, in FIG. 7, V_(pa) is approximately 7.3 V while V_(na) isapproximately −7.8 V. This discrepancy may be characterized by an offsetpotential (V_(off)). While not being bound by any particular theory, theoffset potential may be due to embedded charge created during some ofthe film deposition steps during interferometric modulator manufacture.The offset potential may be simply defined as the average of V_(pa) andV_(na) (e.g., approximately −0.25 V in FIG. 7):V _(off)=(V _(pa) +V _(na))/2Alternatively, the offset potential may be defined as the average of thecenters of the memory windows:V _(off)=[((V _(pa) +V _(pr))/2)+((V _(na) +V _(nr))/2)]/2Those of skill in the art will appreciate other ways of expressing theconcept of offset potential. Although there are two memory windows inthe hysteresis characteristic, for the purposes of driving aninterferometric modulator, a single parameter, ΔV_(mem), may be defined.Advantageously, ΔV_(mem) may be defined so that voltages falling withinΔV_(mem) will be within either of the two memory windows. Thus, forexample, the smallest of the two memory windows may be used forΔV_(mem):ΔV _(mem)=Min[(V _(pa) −V _(pr)),(−(V _(na) −V _(nr))]The bias potential, V_(b), may be defined as the amplitude of thevoltage at the centers of the memory windows. Thus, V_(b) defines thedisplacement of the memory windows from 0 V. Because there are twomemory windows, a single parameter V_(b) may be defined as the averageof the amplitude of the centers of the two memory windows:V _(b)=[((V _(pa) +V _(pr))/2)−((V _(na) +V _(nr))/2)]/2

FIG. 8 depicts another view of the hystersis curve described in FIG. 7.In FIG. 8, reflectivity (normalized optical response) is plotted as afunction of drive voltage. All of the parameters described above aredepicted in FIG. 8. For example, V_(pa) 104, V_(pr) 106, V_(na) 108, andV_(nr) 110 are indicated on the voltage axis. ΔV_(mem) is indicated forthe positive memory window 112 and negative memory window 114. Theoffset potential V_(off) is indicated as a small negative offset 128from 0 V due to asymmetry in the hysteresis curve. Finally, the biaspotential V_(b) is indicated as an offset 130 of the memory window from0 V. In one embodiment, reflectivity may be normalized by normalizingthe average reflectance observed from actuated interferometricmodulators to a fixed positive value (e.g., 0.25 in FIG. 8) and theaverage reflectance observed from undriven interferometric modulators toa fixed negative value (e.g., −0.75 in FIG. 8). The actuation potentialsV_(pa) 104, V_(pr) 106, V_(na) 108, and V_(nr) 110 may then bedetermined as the potentials were the normalized reflectivity is zero.

The parameters described above may be used to determine the appropriatevoltage drive characteristics of the interferometric modulator. Forexample, the offset potential V_(off) may be used to define a DC offsetpotential to be applied to all drive voltages. The bias potential V_(b)may be used to define the amplitude of a voltage waveform (e.g., memorywaveform) that can be applied to the interferometric modulator withoutcausing the interferometric modulator to change state. One use of thebias potential in driving interferometric modulators is described abovewith reference to FIG. 4. Those of skill in the art will appreciateother drive schemes making use of the parameters determined by themeasurement methods described herein. Testing for this purpose may beconducted shortly after release of an interferometric modulator displayor may be conducted periodically during operation of the display toadjust the drive characteristics. For example, an optical detector maybe incorporated within the interferometric modulator display to detectlight reflected from at least some of the interferometric modulators inthe display.

The parameters described above may also be used in testing ofinterferometric modulators for quality control purposes. For example,thresholds and tolerances may be defined for some or all of the aboveparameters that determine whether the tested interferometric modulatorsmay be used in a display. Advantageous characteristics of aninterferometric modulator for use in a display is that the offsetpotential be low, the memory window be large enough to preventaccidental actuation or release, and the bias potential be high enoughto provide separation between the two memory windows. The precise valuesand tolerances required will depend on the specific application of theinterferometric modulators. In one embodiment, a memory function may bedefined as follows:ΔV _(mem)/(V _(pa) −V _(off))In some display applications, it is desirable that the memory functionhave a value around 0.2. If the memory function deviates significantlyfrom the desired value, than the tested interferometric modulators willbe deemed insufficient for use in a display and thus to have failed thetesting. In one advantageous embodiment, an array of interferometricmodulators is tested prior to incorporation with drive electronics sothat if the array fails testing and is discarded, the costs are reduced.

The parameters described above may also be used to determine physicalcharacteristics of the interferometric modulators. For example, theactuation voltages may be related to the tension and spring constant ofthe movable mirror. Thus, parameters obtained in testing may provide anindication of what type of failure has occurred if the hysteresischaracteristics are not adequate.

In some embodiments, methods are provided for testing a plurality ofinterferometric modulators. FIG. 9 depicts a flowchart of possiblemethod steps. Depending on the particular embodiment, steps may be addedto those depicted in FIG. 9 or some steps may be removed. In addition,the order of steps may be rearranged depending on the application. Atblock 150, a time-varying voltage waveform is applied to theinterferometric modulators. As described above, in one embodiment thetime-varying voltage waveform is a triangular waveform. The voltagewaveform may be simultaneously applied to the plurality ofinterferometric modulators. In one embodiment, the plurality ofinterferometric modulators are less than all of the interferometricmodulators in a display. In one embodiment, the voltage waveform isapplied to all interferometric modulators in a display even though onlya subset of interferometric modulators are tested.

Continuing to block 152, the reflectivity of light from the plurality ofinterferometric modulators is detected. Reflectivity may be detectedfrom all interferometric modulators in a display or from only a subsetof interferometric modulators in a display. In some embodiments, aportion of a display is magnified using a microscopic lens for purposesof detecting reflectivity from only that portion of the display.Reflectivity is advantageously measured by exposing the interferometricmodulators to a light source and measuring the intensity of reflectedlight. In some embodiments, intensity of reflected light is measuredusing a photo detector. In some embodiments, a filter is inserted infront of the photo detector to increase the difference in reflectanceobserved between the actuated and non-actuated states.

After reflectivity of light has been measured at step 152, the processmay proceed differently depending on whether the goal of the testing isto determine drive parameters or to test the display for quality controlpurposes. If the testing is to determine drive parameters, the processmay proceed along branch 154. If the testing is for quality control, theprocess may proceed along branch 156. On branch 154, one or moreelectrical parameters are determined at block 158. These parameters maybe any of the parameters discussed above or any other parameter suitablefor use in determining the appropriate drive characteristics for theinterferometric modulators. These parameters may be determined bydetecting the voltages where sudden changes in reflectivity from theinterferometric modulators occur. These voltages may be used directly orother parameters may be calculated using these voltages as describedabove. Alternatively, a hysteresis or similar reflectivity-voltage curvemay be measured and analyzed by an appropriate algorithm. For example,the curve may be fit to a hysteresis model are the voltages wherereflectivity in the curve reaches certain predefined thresholds may bedetermined.

Referring again to branch 156, reflectivity as a function of voltage isdetermined at block 160. In some embodiments, step 160 may includedetermining one or more electrical parameters such as is described abovewith reference to block 158. The determination made at block 160 may beused to either accept or reject the interferometric modulators for usein a display at block 162. The acceptance or rejection criteria may bebased on values and tolerances for specific parameters such as describedabove or based on the general shape of the reflectance-voltage curve,such as by observing the hysteresis characteristic depicted in FIG. 8.

Both branch 154 and branch 156 next optionally proceed to decision block164 where it is determined whether additional interferometric modulatorsare to be tested. The additional interferometric modulators may bemodulators on the same array. For example, a different subset ofinterferometric modulators on a display may be tested. If additionalinterferometric modulators are to be tested, the process returns toblock 152 for detecting reflectivity of light from the additionalinterferometric modulators. If additional modulators do not need to betested, the process ends at block 166.

In one embodiment, the sequence depicted in FIG. 9 of steps 150-152-158or steps 150-152-160 may proceed according to the flow chart depicted inFIG. 10. At block 150 in FIG. 10, a time-varying voltage is applied asdescribed in FIG. 9. At block 170, reflectivity of light from theinterferometric modulators is measured and, simultaneously, the voltageapplied to the modulators is measured. At decision block 172, it isdetermined if a sudden change in reflectivity has been observed at anytime during the application of the time-varying voltage. The absence ofa sudden change in reflectivity may indicate that the applied voltagehas not reached high enough levels to induce actuation of theinterferometric modulators. Thus, if no sudden change in reflectivity isobserved, the process proceeds to block 174 where the maximum voltage ofthe time-varying voltage is increased and the process starts over atblock 150. If a sudden change in reflectivity is observed at decisionblock 172, the process proceeds to block 176, where the average measuredreflectance from interferometric modulators in an actuated state isnormalized to a predetermined, positive fixed value. Next the measuredreflectance from the interferometric modulators in an undriven (e.g., 0V) state is normalized to a predetermined, negative fixed value at block178. Finally, at block 180, actuation and release potentials aredetermined by determining the potentials where the normalizedreflectance crosses zero values. The testing process may then proceed asin FIG. 9 to block 164 or block 162.

In one embodiment, the time-varying voltage stimulus is applied in aperiodic fashion. Thus, for example, a series of multiple triangularwaveforms may be applied to the interferometric modulators. The seriesof waveforms may be used to detect a series of hysteresis curves. Thesehysteresis curves may be used to calculate multiple parameters to whichstatistics may be applied to determine driving parameters or to makequality control determinations. Alternatively, the multiple hysteresiscurves are averaged to increase the signal-to-noise ratio prior todetermination of parameters.

In some embodiments, the testing process may be automated. Thus, forexample, detection of reflectivity as a function of applied voltagestimulus may be automatically performed at pre-determined areas on aninterferometric modulator array. The calculation of parameters andquality control determinations may be automatically performed usingsuitable algorithms executed on a computing device. Furthermore,positioning of interferometric modulator arrays within a testingapparatus may be automated so that high throughput of mass-manufacturedinterferometric modulator displays may be accomplished. In someembodiments, a selected percentage sample of mass-manufactured displaysare tested for quality control purposes.

One embodiment of an apparatus suitable for performing the abovedescribed testing is depicted in FIG. 11. The interferometric modulatorarray 200 is electrically connected to a voltage driving source 202. Thevoltage driving source 202 applies the time-varying voltage stimulus,such as a triangular voltage waveform, to the array. The voltage signalmay be applied to all interferometric modulators in the arraysimultaneously. Alternatively, a voltage signal may be applied to onlythose interferometric modulators from which reflectivity are beingmeasured. A light source 204 provides illumination of theinterferometric modulators. In one embodiment, a standard D65 lightsource is used for the light source 204. Light source 204 provides light206 to the interferometric modulator array 200, which is then reflectedupward. A photo detector 208 may be used to detect the intensity oflight 206 reflected from the interferometric modulator array 200. Adiffuser film 210 may be optionally placed over the interferometricmodulator array 200. The diffuser film 210 scatters the light 206transmitted into and reflected from the interferometric modulator array200. Such scattering allows the light source 204 and detector 208 to beplaced at angles 212 and 214 relative to the array 200. Whilereflectivity from the array 200 may be at a maximum if angles 212 and214 are complementary, the use of a diffuser film 210 allows fordetection off the specular angle. If a diffuser film 210 is not usedthen it is advantageous that light 206 enter and reflect back from thearray 200 at an angle close to perpendicular to the array 200. Such aconfiguration is desirable because interferometric modulators typicallyhave a narrow viewing angle causing the intensity of reflected light tofall rapidly at wider angles. A computer 216 running appropriatesoftware may be used to record reflectivity versus voltagecharacteristics (e.g., the hysteresis curve) and calculate electricalparameters. The computer 216 may also be used to control the automatedtesting of interferometric modulators.

FIG. 12 depicts an apparatus that allows in-line lighting and detectionfor reflectivity measurements of interferometric modulators without useof a diffuser film. In the configuration of FIG. 12, the angles 212 and214 identified in FIG. 11 are effectively reduced to zero. In FIG. 12,light source 204 provides light 206 to a beam splitter 220. A portion oflight 206 is reflected from beam splitter 220 down onto theinterferometric modulator array 200, which is being driven by voltagesource 202. A portion of the light 206 reflected back from the array 200passes through beam splitter 220 and into photo detector 208, which maybe connected to a computer 216 as described above. Optionally, amicroscopic lens 222 may be used to focus the incident light anddetection area on a portion of the interferometric modulators in array200. Those of skill in the art will recognize other means for achievingin-line lighting and detection. For example, a bundle of fiber optics,some of which provide incident light and others which detect reflectedlight may be aligned over the desired area of array 200.

FIG. 13 depicts an interferometric modulator array 200 containing aplurality of interferometric modulator elements 250. As discussed above,a microscopic lens may be used to focus detection of reflectivity on aportion of the interferometric modulator array 200. Thus, for example,area 252 may be the area that is detected during testing. The area 252tested may be of any suitable size. In one embodiment, only a fewinterferometric modulator elements 250 are included. In one embodiment,an approximately 1 mm diameter spot is measured. In some embodiments,multiple areas, such as areas 252, 254, 256, 258, and 260 are measuredsequentially on the same array 200. The number of areas and location ofthe areas may be selected based on the desired testing standard. Forexample, the suggested number of spot measurements and their locationsrecommended by ANSI or VESA for display testing may be used. In oneembodiment, a single area 252 near the center of the array 200 ismeasured.

In one embodiment, a color interferometric modulator is tested. In thisembodiment, it is advantageous to test reflectivity from each colorseparately. Thus, each set of sub-pixels may be driven separately andreflectivity separately measured. For example, all red sub-pixels in adisplay may be driven with the time-varying voltage stimulus while thegreen and blue sub-pixels are driven to an actuated (e.g., dark) state.Thus, reflectivity from only red sub-pixels is measured. The separatehysteresis characteristics of the blue and green sub-pixels may besimilarly measured.

Although the foregoing examples have been described in terms of certainembodiments, other embodiments will be apparent to those of ordinaryskill in the art. Additionally, other combinations, omissions,substitutions and modification will be apparent to the skilled artisan,in view of the disclosure herein. The scope of the invention is limitedonly by the following claims.

1. A method of determining electrical parameters for driving a pluralityof interferometric modulators, comprising: applying a time-varyingvoltage stimulus to a plurality of interferometric modulators which arestable in an actuated state and a non-actuated state, wherein thetime-varying voltage stimulus includes positive voltages higher than apositive actuation potential of the interferometric modulators andnegative voltages lower than a negative actuation potential of theinterferometric modulators; detecting reflectivity of light from theinterferometric modulators; and determining one or more electricalparameters from said reflectivity of light in response to saidtime-varying voltage stimulus, the electrical parameters comprising anamplitude of an offset voltage in the interferometric modulators.
 2. Themethod of claim 1, wherein determining comprises determining a voltagenecessary to cause the interferometric modulators to change from thenon-actuated to the actuated state.
 3. The method of claim 1, whereindetermining comprises determining a voltage necessary to cause theinterferometric modulators to change from the actuated state to thenon-actuated state.
 4. The method of claim 1, wherein determiningcomprises determining amplitude of a bias voltage in the interferometricmodulators.
 5. The method of claim 1, wherein determining comprisesdetermining amplitude of a memory window in the interferometricmodulators.
 6. The method of claim 1, wherein determining comprises:normalizing the reflectivity to one or more pre-determined values; anddetermining potentials where the normalized reflectivity crosses zero.7. A system for testing a plurality of interferometric modulators,comprising: an illumination source adapted to provide incident light toa plurality of interferometric modulators; a voltage source adapted toapply voltages to a plurality of interferometric modulators which arestable in an actuated state and a non-actuated state, the voltagesincluding at least one positive voltage higher than a positive actuationpotential of the interferometric modulators and at least one negativevoltage lower than a negative actuation potential of the interferometricmodulators; an optical detector adapted to detect light reflected fromthe plurality of interferometric modulators; and a computer adapted todetermine one or more electrical parameters from said detected light,the electrical parameters comprising an amplitude of an offset voltagein the interferometric modulators.
 8. The system of claim 7, wherein theillumination source provides incident light at an angle that issubstantially perpendicular to the interferometric modulators.
 9. Thesystem of claim 7, wherein the voltage source is simultaneouslyelectrically connected to all interferometric modulators in a reflectivedisplay.
 10. The system of claim 7, wherein the voltage source isadapted to apply a time-varying voltage waveform.
 11. The system ofclaim 10, wherein the voltage source is adapted to apply a triangularvoltage waveform.
 12. The system of claim 7, wherein the opticaldetector is a photo detector.
 13. The system of claim 7, wherein theoptical detector is adapted to detect light from less than allinterferometric modulators in a reflective display.
 14. The system ofclaim 7, wherein the optical detector is adapted to detect light at anangle that is substantially perpendicular to the interferometricmodulators.
 15. The system of claim 7, wherein the optical detector isadapted to detect light through a diffuser.
 16. The system of claim 7,further including a filter disposed in front of the optical detector.17. A method of testing a plurality of interferometric modulators,comprising: applying a time-varying voltage waveform to a plurality ofinterferometric modulators which are stable in an actuated state and anon-actuated state, wherein the time-varying voltage waveform includespositive voltages higher than a positive actuation potential of theinterferometric modulators and negative voltages lower than a negativegoing actuation of the interferometric modulators; detectingreflectivity of light from the interferometric modulators; repeatingsaid applying and detecting steps one or more times; averaging at leasta portion of the detected reflectivity; and determining one or moreelectrical parameters from said averaged reflectivity, the electricalparameters comprising an amplitude of an offset voltage in theinterferometric modulators.
 18. A method of testing a colorinterferometric modulator display, the display comprising a plurality ofsub-pixel types, each sub-pixel type corresponding to a different color,the method comprising: applying a time-varying voltage waveform to onesub-pixel type, wherein the time-varying voltage waveform includespositive voltages higher than a positive actuation potential ofinterferometric modulators in the display which are stable in anactuated state and a non-actuated state and negative voltages lower thana negative actuation potential of the interferometric modulators;detecting reflectivity of light from said sub-pixel type; determiningone or more electrical parameters from said detecting, the electricalparameters comprising an amplitude of an offset voltage in theinterferometric modulators; and repeating said applying, detecting, anddetermining steps for another sub-pixel type.
 19. A system for testing aplurality of interferometric modulators, comprising: means for applyinga time-varying voltage waveform to a plurality of interferometricmodulators which are stable in an actuated state and a non-actuatedstate, wherein the time-varying voltage waveform includes positivevoltages higher than a positive actuation potential of theinterferometric modulators and negative voltages lower than a negativeactuation potential of the interferometric modulators; means fordetecting reflectivity of light from the interferometric modulators; andmeans for determining one or more electrical parameters based on saiddetected reflectivity, the electrical parameters comprising an amplitudeof an offset voltage in the interferometric modulators.
 20. The methodof claim 19, wherein the means for applying a time-varying voltagewaveform is a voltage source.
 21. The method of claim 19, wherein themeans for detecting reflectivity is a photo detector.
 22. The method ofclaim 19, wherein the means for determining one or more parameters is acomputer.